Amplitude limiter circuit



May 6, 1947. W, R, KOCH 2,420,248

AMPLITUDE LIMITEE CIRCUIT I Filed July 19, 1944 4 Sheets-Sheet 2 l p A?,fia, gaar'- ,e 353050 -ff- INVENTOR. W//vF/z-'La P. Koch' I BY May 6,1947. w. R. KOCH 2,420.248

AMLITUDE LIMITER CIRCUIT Filed July 19, 1944 4 sheets-shea a ,FEE 4 V04rags /X May s, 1947. W,R KOH' 2,420,248

I AMPLITUDE LIMITEE CIRCUIT Filed July 19, 1944 4 Sheetst-Sheet 4 ull!AAAAAAA 0i' E E? l 4f ZO INVENTOR.

WWF/5L@ E. Moc/7 rmAQ/vy Patented May 6, 1947 2,420,248 AMPLITUDELIMITEE `CIRCUIT Winiield R. Koch, Haddonfield,

Radio Corporation of America,

Delaware N. J., assignor to a corporation of Application July 19, 1944,Serial No. 545,664 6 Claims. (Cl. Z50-20) My present invention generallyrelates to amplitude limiting systems, and more amplitude limitercircuits adapted for use in receivers of angle modulated carrier waves.

In the reception of angle modulated carrier waves it is necessary thatthe demodulator of ,the receiver be presented with modulated carrierwave energy Whose amplitude variations are an accurate representation ofthe angle modulations of the Vreceived waves. This is insured byemploying an amplitude limiting device -prior to the discriminatorsection of the demodulator to prevent undesired amplitude variations inthe received'angle modulated Waves from causing la response in theoutput of the demodulator. The generic term angle modulated is to beunderstood as including frequency modulation (FM), or phase modulation(PM), or hybrid modulations having characteristics of both FM and PM. Byway of specific illustration the present description will be explainedin connection with a system for receiving FM carrier waves, although myinvention is not restricted to such reception.

One of the most widely known andV used forms of discriminator-rectifiercircuits is that disclosed and claimed by S. W.4 Seeley in his U. S.Patent No. 2,121,103, granted June 21, 1938. As commonly used inaccordance with the Seeley patent, the discriminator section comprises apair of coupled resonant circuits each tuned to a predeterminedoperating frequency (usually lthe intermediate frequency of a receiverof the Widely-used superheterodyne type). The amplitude limiter tubeusually includes in the plate circuit thereof vthe primary of the twocoupled circuits, while a pair of opposed rectifier devices areconnected to opposed ends of the secondary circuit coil which iscenter-tapped. The rectified volttages of the rectier devices arecombined in polarity opposition, and the resultant voltage is accuratelyrepresentative of the frequency variations of the received FM waves.

The well known frequency deviation vs. voltage output characteristic of.the Seeley demodulator system is generally an inclined, approximatelylinear characteristic whose upper and lower ends are bent in oppositedirections. The linearity of the characteristic depends upon the degreeof coupling between the primary and secondary circuits of thediscriminator section, and the effective selectivity of these circuits.Usually the coupling is approximately critical coupling, under whichcondition the primary circuit voltage response has a pronounced doublehump.

Unlike the normal I. F. transformers preceding the limiter tube, theresponse curve of the priparticularly to 'l mary of the discriminatortransformer is of large i importance. The pronounced, double-humpresponse of theV primary provides a correction for inherent centralcurvature in the normal frequency deviation vs. voltage outputcharacteristic oi the tdemodulator.

The amplitude limiters employed in the past with the Seeleydiscriminator, or more generally any discriminator Whereinthe primaryone of coupled tuned circuits possesses a doublehumped response curve,have generally been so constructed as inherently to alter under varying.conditions of operation the optimum response curve of the discriminatorprimary circuit. For example, in one form of prior amplitude limitercircuit the object has been to maintain a constant signal voltage at theplate of the limiter tube. The effect of such aV constant voltage sourceacross the discriminator primary circuit is effectively to shunt thelatter by a substantially zero impedance. Y As a consequence theresponse curve of the primary circuit is flattened out under conditionsof operation which will hereinafter be further described, and thevoltage response characteristic ci the secondary may approach that of asingle tuned circuit. Because of this the linearity and length of thediscriminator characteristic are reduced. Hence, the advantages of apair of coupled tuned circuits are largely lost. The frequency deviationvs. voltage output characteristic will depend on the strength of thereceived FM waves, and for strong signal reception the characteristic`will have a shorter length, and be of increased slope, than for Weaksignals.

It may, therefore, be stated that it .is an important object of mypresent invention to provide an improved amplitude limiter, and, moreparticularly, an amplitude limiter which affords an output of uniformamplitude and which does not deleteriously `affect the operation of thesucceeding discriminator. More speciiically, one of my objectives is topreserve under varying conditions of operation the desirabledouble-humped response curve of the primary circuit of the discriminatortransformer by feeding FM Waves thereto through an amplitude limitertube circuit which possesses a substantially constant plate currentcharacteristic constant plate voltage characteristic.

Another important object of my invention is to provide an amplitudelimiter tube of the screen grid type, wherein the control grid andscreen grid voltages are varied at a relatively fast rate in response toapplied signals so as to maintain the plate current flow substantiallyconstant over a range of signal voltage amplitudes above a predeterminedvalue.

Another object of my invention is to provide an amplitude limiter whosetuloe has the plate thereof operated at a positive direct currentvoltage of relatively high value, While the tube is provided with ascreen grid whose positive direct urrlll voltage is lower than the platevoltage in contradistinction to aV Another object of my invention is toimprove the efficiency and construction of FM receivers by improving theamplitude limiter operation and Y preventing the limiter from adverselyaffecting the desired response characteristic of the discriminator.

A more specific object of my invention is toY provide an amplitudelimiter tube whose signal input grid circuit and screen grid circuiteach include resistorcondenser networks of relatively short timeconstants, and whose plate circuit contains only very small directcurrent impedance.

A further object of my invention is to provide an amplitude limiter tubeof the type including a screen grid having means in circuit therewith topermit it to vary over a wide range oi positive voltages thereby toco-operate in maintaining a substantially constant plate current, andother means being operative to prevent the screen Voltagefrombecomingtoo low and causing the gain of the tube to fall for weaksignals.

Still other objects of my invention are to improve amplitude limitercircuits for FM reception, and more particularly to provide alimiter-discriminator network of desirable characteristics.

Still other features of my invention will best be understood byreference to the following description, taken in connection with thedrawing, in which I have indicated diagrammatically several circuitorganizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows one embodiment of the invention;

Fig. 2 shows idealized response curves of the discriminator transformer;

Fig. 3 illustrates typical discriminator charac'- teristics, both withand without the invention;

Fig. 4. s hows limiter characteristics both cal- 'culated andexperimental;

Figs. 5a, 5b,Y 5c show of the limiter tube for signals respectively;

Fig. 6 illustrates graphically the effect of varygraphically theYoperation week, medium and strong 'ing the value of R in the screengrid circuit of the limiter tube;

Fig. '7 shows a modification of the system of Fig. 1, and

Fig. 8 illustrates a further modification of Fig. 1.

Referring now to the various figures ofthe accompanying drawings,wherein like reference letters and numerals in the different figuresdesignate similar elements, there is shown in Fig. 1 so much of an FMreceiver as is essential to a proper understandingA of my invention. Itis assumed that the limiter and demodulator of Fig. 1 are embodied in asuperheterodyne receiver, since that form of receiver is most widelyemployed at present. The customary selective radio frequency amplifier,converter and I. F. amplifier precede the input transformer I whichfeeds the I. F. signals to the signal grid 2 of limiter tube 3. Thoseskilled in the art of radio communication. and more specifically FMcommunication, are fully aware of the details of circuit design prior tolimiter tube 3. The received FM waves may havea carrier or centerfrequency in any of the known frequency bands allocated to FM or vPMreception. The present FM Abroadcast range extends from 42 to 50megacycles (mc.)

Assuming operation in the 42-50 mc. range, the

selector circuits of the receiver between the antenna and discriminatornetwork will each be designed satisfactorily to pass a band offrequencies of the order of 200 kilocycles (kc.) This relatively wideband pass selector characteristic is required, Ibecause in compliancewith present standards of FM broadcasting the maximum fre- `swing of aselected modulated carrier.

Y quency deviation at each FM transmitter is up to 75 kc. on either sideof the normal carrier frequency. As pass band width of 200 kc. insuresthe acceptance of the overall kc. frequency The frequency variations ofthe signal energy are, of course, representative of the modulationapplied to the carrier wave at the transmitter, the extent of thefrequency variation or deviation being proportional to the amplitude ofthe modulating signals while the rate of deviation is dependent on themodulation frequencies per se. Since a PM wave essentially differs froman FM wave in that the extent of frequency deviation is proportionatelyhigher for the higher modulating frequencies, it will be clear that theFM receiver may be employed for detection of PM waves with de-emphasiscorrection subsequent to the demodulator.

Assuming, now, that there has been applied to the primary circuit 4 ofI. F. transformer l the FM energy whose center or carrier frequency hasbeen reduced to the I. F. value of 4.3 mc., each of circuits 4 and 5 istuned to the I. F. value. The circuits `4 and 5 are coupled to provide asubstantially band pass response characteristic about 200 kc. wide. Aspreviously stated, this is also true of the selector circuits prior totransformer l. Limiting is accomplished by means of tube 3, shown as apentode tube by way of specific illustration. 'Ihe cathode 6, eitherindirectly or directly heated, is preferably connected directly toground, while the low potential side of input circuit y5 is connected toground by a resistor R1 shunted by condenser C1. The magnitudes of thesecomponents R1 and C1 are so chosen as to provide a relatively short;time constant. For

' example, and in no way restrictive, R1 may be 150,000 ohms while C1mat7 be 22 micro-microfarads. The control grid 2 is connected to thehigh alternating potential side of circuit 5.

The plate or anode 1 of limiter tube 3 is connected to operate at arelatively fixed direct current potential. Thus, the plate 1 isconnected through the inductance 8 of the primary circuit P of thediscriminator network to a point on a direct current source (not shown).having a potential of, for example, +250 volts. It will be observed thatbetween the +250 volts point and the plate 1 there is no material directcurrent resistance, the direct current resistance of coil 8 .being verysmall. This is the reverse of the construction of limiters well known inthe prior art, wherein substantial resistance is deliberately introducedinto the plate circuit to reduce the normal plate voltage to such a lowpositive value that the tube Will readily saturate. The screen grid 9 oftube 3is connected to the +250 volts point through a. resistor R whoseupper end is connected to ground by condenser C. The values of R and Care so chosen that network R, C has a relatively short time constant. Byway of specific example, and in no way restrictive, R may have a valuechosen from a range of 33,000 ohms to 120,000 ohms. The condenser C mayhave a value of the order of 68 micro-microfarads. The normal no-signalvoltage of the screen grid 9 will be relatively small compared to the.plate voltage. E'or example, the no-signal screen voltage may be as lo,was -L-'IDf volts. However, during s'gn'al reception' the volt.- age of.the screen will vary atv a relatively rapid rate, by virtue of the shorttime constant of network RI., C'. Here, again, the presentY limiterconstruction is. the reverse of" that employed in the prior art, sincein the.V past' it has usually been proposedto maintain the screenvoltage" invariable throughout signall reception while permitting. theplate voltage to vary. The suppressor grid' I U. isconnectedl to thecathode 6 within the' tube envelope, and .performs its usual functionYoil suppressing, secondary electron emission from plate 1.

. The coil's and I I are of the respective primary and. secondary`windings of the di'scriminat'orA transformer. Coil 8 and condenser 3provide the resonantprimary circuit P, while the secondary coil 'II and'condenser II in parallelv with it provide the resonant 'secondarycircuit S. Each of circuits P and' S is tunedlto the operating I. F.value of' 4.3 mc.. These circuits are preferablyv coupled so as toprovide a. substantially doublehumped response curve for the' primarycircuit P, while providing arband pass curve for the second'ary circuit.The high alternating potential side oiprimary circuit Pis connected tothe'mid'- point of coil II" through a direct current blocking. condenserI2 which functions as a direct connection so far asthe I. F. currentsare concerned. In other Words condenser I2 impartsv no phase shift tothe I. F. currents applied to themidpoint of coil I'I`.

The circuitsP and S constitute a discriminator network. oi the typedescribed inthe aforesaid Seeley patent, and referred to herein asaSeeley discriminator. It is relatively widely used'in FM receivers, andits functions are well-known to those skilled in tion. It is sufiicientfor rthe purposes of the present application to explain that at theinstant` when the appli'edll.. F. energy hasa fre-V quency equal to theresonant frequencies of circuits P and S, then the signal. voltageacross circuitn S will vbe 90 out of phase with the Voltage acrossprimary circuit P. This is due to the magnetic coupling between resonantcircuits P and VS'. However, the connection including condenser I`2 willalso inject into the circuit S- primary signal; voltage which has not'been subjectedtoanyphase shift. l y

Due to the fact that the primany voltage is applied to the mid-point ofcoil II, it will' be seen that from each end( of coil I`I to groundthere will exist primary signal voltage in phase quadrature .with theinduced phase-shifted signal voltage, the induced voltagesin each halfofthe secondary coil II being of opposite polarity. Hence, there willexist between each end of coil It and ground a resultant voltage whichisthe vector sum of the phase quadrature-related voltages across each.half of secondary winding II and the primary voltage. These resultantvoltages will be of equal magnitude at the instant when the I. F. energyapplied to circuit P i's equal. to the resonant frequencies of thediscriminator circuits. However, should the instantaneous signalfrequency deviate or shift with respect to the, predetermined referenceire quencies of' circuits P and S, then the resultant voltages at theopposite ends of coilr II will' become unequal because of phase changesaway4 from the quadrature phase relation. The inequality wil'l bedependent upon the magnitude the art of. radio communicaoff frequencyydeviation f from the,I vrefe-rence free quencyt; while the'directionfoff the inequality-i will depend upon the directionof'frequency' d"e4 viati'on. In this way' the frequency-variable wavesareV translatedJ orjtransformed into -a-*pair of vvoltages whichy areequal in' magnitude at the instant when the signalfrequency is equal tothev discriminatorreference frequency; but vvl'iicli;`

vary with respectr to each otherfor-"frequencyk deviations fromv the:center-frequency.'n The "func-` tion` of the tube I31i's-toprovidea pairoffrec tier devices for recti'lfying the; aforesaid-"pail-fof'variablevoltages; i f

Tubel I3, while shown-io`y'wa1y-of exampleas a- (5l-I6- type tubeembodying Va pair'of separate dit. odes, may be'repla-ced' by a pair ofindependent diode tubes or other'suitable rectiersf.V The. anode HI'.and cathode I5}o f the-upper" diode device 'aref connected' between theupper sideof circuit S-and the upper end of loadresistor I6. The lowerdiode device II, I8 is connected between the -grounded end of' loadresistor IS'and the lower side of 'cire' cuit Si Thefmidpoint ofsecondary coil I Is i'scon#l nected by I. F. choke coil 2Q tothevjunction of load: resistors I-Sand ISP.y Hence, theresultant I. Flvoltage' applied to-anodevI4iwil1ibe rectified', and'- the rectifiedvoltage will appear'4 across resistor I6. In the same way theresultantvI. F. voltage applied to diode anode I'I will be rectied, and' therectified voltagel will appear across resistor I9. Since the rectifiedvoltages across resistors"|6f and I9 are combined in polarityAopposition rela-1 tive to ground,`the resultant potentialvv at'l theendA of resistor' I6 connected to cathode I5?V will be-Zeroat thelinstant when the frequency'of the lpplied FM- Waves is eqrialv to theyreference frequency of the discriminator' circuits P and S, andv willvary* in magnitudeV and polarity depending upon the extent and'directionof frequencydeviation.` 'I-he resistors lila" and I9-are each bypassed`by suitable- I. F; lbypass condensers, and the modulation frequencycomponent of the resultantfrectiedvolt'- age is applied through coupling-condenservZIf to anysuitable audio frequency Vmodulation.ampliner-network.

In Fig. 2` there are shown the primary-'and sec. ondary responsecurves'Pi-'andjS'i-of circuitsland. S- respectively. 'Ihese curves aresecuredv by plot-1 ting frequency" as abscissa against response asiordinate. Asexplained previously, itrislhighly desirable to have theresponse curve Pi markedly double-bumped. The secondary response curveSri`s relativelyv flat-topped with somewhat ofl a depression-atitscenter. In practice ythe trans'-A former windings 8 and II= are coupledapproximately to the critical coupling value, .which .provides the curvePt. Itshould be noted that the primaryis. coupled' to the` diodes. I4,I5- and II,4 I8 both through, the transformer windings andithroughcondenser I2'. Because of the connectionincluding condenser IZ,the response curve of the primary circuit P.(as well as the secondarycircuit Si) is important tothe operation of the discriminator. Thefbestoverall` discriminator characteristic, bothJ as to` linearity andAlength, is ob tai-ned when the response o'f'primary P issubstantially-in accordance with the double-humped curveP'i in Fig. 2. vY Y Fig, 3 the-curve D relates frequency as abscissaagainst'- voltageoutput vas ordinate.'- Theportion of curve'Dloetween the peaks thereofis substantially linearL by virtue of the correction introduced by thedouble-humped primary response curve Pi. If, now,V there occurs 'anyflattoning ofthe primary' response curve, say to Pi (Fig. 2), `the eiectwill beto change the overall discriminator characteristic from D towardor beyond the curve D (Fig. 3). Such a change would be highlyundesirable, since it shortens and otherwise deleteriously affects thelinear portion of the discriminator characteristic.

Returning now to the limiter tube, the known arrangements of the priorart wherein a constant signal voltage is maintained at the limiter plate1 may cause upon reception of signal energy of varying intensitiesapproximately the change in primary response curve from P1 to P1'. 1This may be explained as follows. If the screen grid voltage ismaintained invariable, Vand the direct current plate voltage ispermitted to vary in order to provide a constant signal voltage outputfrom the primary circuit, there is provided the equivalent of a zeroimpedance across the primary winding 8. -That is, no matter whatimpedance is put in the plate circuit of tube 3 thevoltage across itwill lbe the same. Since this means in effect that the primary 8 isbeing shunted by a very low resistance, the eiect is greatlyto dampenthe primary response curve. Hence, the desirable doublefhumped curve P1may in the reception ofv strong signals be changed to the relativelyattened response curve P1.V At the same time the selectivity due tocircuit P will largely be cancelled, and the tendency will be for thesecondary circuit S alone to supply selectivity whereby thediscriminator transformer will act like a single tuned circuit with arounded top, instead of the relatively flat top S1 of Fig. 2. If thevoltage across the primary of the discriminator transformer were heldconstant, the overall discriminator characteristic would depend on thestrength of the received signal, would be narrow for signals strongenough to cause limiting, and might actually change from curve D to oneeven Worse than D'. If, on the other hand, as in the use of myinvention, the limiter plate voltage is kept high so that it neverswings over the knee of the plate-current vs. plate-voltagecharacteristic, the discriminator characteristic will remain like D.

What is desired, however, is a constant signal frequency current Vin theplate circuit of the limiter tube. This corresponds to a very high plateresistance of the limiter tube. If this is many times as high as theimpedance of the primary circuit P, as is 4usually the case, then theoverall discriminator characteristic D will be the grid at the precedingtube. My present view is same for weak signals as for strong signals.VFurther, the linearity of the characteristic will be preserved, sincethe desirable double-humped re..

sponse curve P1 will be maintained and will not be degenerated toresponse curve P1. l

I secure the desired limiter action by the relatively simple arrangementshown inFig. 1. The resistor R in the screen voltage lead landassociated condenser C cause the screen voltage to vary at a fast rateso that carrier amplitude variations due to amplitude modulation willvary the electron Ilow to the plate 'l in a manner to maintain asubstantially constant signal frequency plate current. In Fig. 4 I haveshown four curves secured by plotting voltage on grid of tube pre-Vceding limiter as abscissa against voltage applied to diodes asordinate. Curve I is a calculated curve using an idealized tubecharacteristic with an invariable screen voltage, but `with R1=10megohms. In other words, R is not in circuit with the screen grid 9, andthe condenser C is removed. Also Yno plate limiting impedances areemployed. The calculated curve shows that the output Vof the limitertube dropsy appreciably as the signal voltage rises above the thresholdvalue of 0.02 volt on the,"preceding grid. Curvek tial droop in thehorizontal part of the charac-" teristic. The eiiectof introducing`suitable resistance, shunted by suitable capacity, into the screen gridcircuit is shown by curve 4, Here the resistor R is given a value of33,000 ohms, and C is chosen of the order of 68 micro-microfarads; R1 is150,000 ohms and C1 is 22 micro-microfarads. The limiter characteristicis now ilat above the signal input voltage of 0.04 volt on the thatwhathas been done is thatthe screen grid has been given a rapid timeconstant so that it functions in the manner of a fast-acting auto-`matic gain control element and provides self,- regulation of the limitertube, augmenting the fast-acting gain control action of the grid 2caused by network R1 and C1 inthe grid circuit. A point involved in myinvention is that grid circuit limiting alone does not give a Vuniformoutput from the limiter-discriminator circuits for diiferent intensitiesof applied signal voltages. Plate circuit limiting when used with gridcircuit limiting changes the discriminator'characteristic. By includinga fast time constant resistor-condenser combination in the screen gridcircuit, the limiter can be made to give a uniform output, and thediscriminator characteristic can be maintained without change at varyingsignal intensities.

The magnitude of screen resistor R determines the threshold or limitingpoint of the limiter tube. In order to provide limiting action at as lowan input vvoltage as possible, a low normal screen voltageis desired.This necessitates a relatively high value of R. Hence, in Fig. 6 I haveshown a family ci?v applied signal voltage vs. effective limiter outputcurrent curves to show the eiTect on the limiting point of the magnitudeof R. At lower initial or normal operating voltages of the screen grid,say +70 volts for example, thev limiting will commence at lower signalinput voltages. It is desirable, therefore, to use higher values (say ofthe order of 120,000 ohms) for R when receiving weak signals than whenreceiving strong signals. There could be provided an adjustment'for thevalue of R so as to permit the set operator to adjust the limiting pointdepending upon the reception area he finds himself in. It will be notedfrom, Fig. 6 that the value of R may be chosen in order to provide theflattest limiting characteristic, or a second value may be chosen whichwill start limiting sooner for weaker signals, but which will not bequite so flat.

Since it is highly desirable to provide the networks R, C and R1, Crwithas short time constants as possible, it may be found under someconditions that condensers C andC1 donot have suiciently low impedancesforroptimum operation. In Fig. 7 I have shown means enabling use of atime constant'of each of R, C and-R1, C1 while using condensers C and C1which might otherwise present more than the desired impedance. Each ofcondensers C and C1 is shunted by a series-resonant `circuitconsistingof a coil ing weak signal inputs.

30 Aand .39 and condenserv3| and 3| respectively,

each tuned to the operating I, F. value. These respectiveseries-tunedcircuits 30, 3l and '30','31 provide a very low impedance for the I. F.current. The condensers C and C1 are necessary Vto bypass harmonics,.butcan -be of smaller capacvthe gain 'for weak, vor low amplitude, inputsignals. VBy employing diode 40, shown in Fig. 8, in association with.screen grid 9 it is possible vto use a higher value for screen resistorR than would normally be the case without the diode.

The higher the value of R the lower the value of signal input voltagefor which limiting begins.

Diode l0 has its cathode connected to the screen grid end of R while itsanode is connected -to a suitable point E2 on potentiometer resistor 4lconnected between the -l-B voltage lterminal and ground. Hence, point 42is normally less .positive than the +B end of resistor 4l. A condenser43 bypasses point 42 and the diode. anode to ground for I. F. currents.The signal grid network R1, C1 is shown in an electrically equivalentarrangement relative to that shown in Fig. 1. The diode 4B is normallyconductive for weak signal input to circuit 5. Hence, the voltage ofscreen 9 will be at the potential of point 42 dur- I-Iowever, when thein- .put voltage at signal grid 2 becomes stronger the screen currentthrough R becomes less, with the result that the cathode of diode i0 mayfor rapidly recurring potentials become more positive than itsanode,inasmuch as the latter is grounded `for I. F. currents, therebydisconnecting the screen grid 9 from point 42. The full voltage acrossthe series resistor R is, therefore, applied to the screen grid, and thelimiter tube from this point on acts as it does inthe circuit of eitherFig. l or Fig. 7. Diode 40, therefore, provides ya controlled minimumscreen voltage 4for the .limiter tube, but withoutaffecting the platevoltage.

Regardless of whether or not the modifications of Figs. '7 and 8 areused, thelimiter `circuit of my present invention `functions to .providea substantially uniform and constant signal frequency current in theplate circuit of the limitertube. This corresponds to a very highplate-resistance of the limiter tube. It is preferred that theVplateresistance of the limiter .be many times as high-asthe impedanceof primary circuitl P. In .this way the desirable response curve P1shown in Fig. 2 will be maintained.

In Figs. a, 5b and -5c `I have graphically 'and 'ideally shown themanner in which the limiter .grid '2 of limiter tube 3. The signal gridis normally at ground potential. Upon the signal energy charging thegrid 2 positive during the irst few positive cycles thereof, gridcurrent ows and .10 produces a voltage across R1 thereby chargingcondenser C1 negative. This negative bias, indi- 'cated `by broken lineX, `offgrid 2 determinesethe operating point'of the tube. 'As the signalpasses Ythrough its alternate negative 4'and positive cycles, the screenand'plate rcurrents veach will follow the "curve b Ias to contour, `th'eplate current, in the -tubes which 'I have .used being, however, of the'order of fourtimes the sCreen'grid'Current.

Upon 'the vsignal intensity increasing to a', for example as showninFig. 5b, the negative'bias for grid 2 'increases "tothepoint wherethetube characteristic looks like a .class B "-a'mplier characteristic.4"I'h'e positive half o'i each signal wave produces a rectied plate'currentpu'lse b. Sincethe 'time'constantof R1, C1isfast,"thebiasjongri'd2 'will 'quickly .follow the signal yamplitude variations, i.e.,wil1becomemore negative as "indicated in Fig. 5b by the 'line .Xvbeing'movedfurther'tothe left. The :fast-acting control network R, Cwill functionto provide a gain control 'voltage Ydue to screen current.flow decrease which tends 'toreduce'the dropin'resstor R, causinghigher screen voltage which in turntends toprevent areduction ininstantaneous plate current now. This eiect is also shown in Fig. 5bwhich 'indicates that .such increase in the positive screen voltage'raises 'the plate current curve from the Kfull -line A -to the brokenline B. 'Ihis allows .theplate current upon 'a ,positive half-cycle ofsignal voltage tobe 1in accordance with the brokenline b" IinFg. 15binstead of the shaded area which shows what the-operation would be ifthe resistor R were not present in the screen grid circuit. -ThisYfurther ymeans that the signal `frequency current inthe .plateciiu cuitis increased yfrom that Vcorresponding to vcurve 3 ofFig. '4 Vto thatcorrespondingto curve 4 ofthat figure. In Vother words, eachfast changeinscreen voltage :causes the tube to operate along a new characteristicof the family of ,possibleplate current-control grid `(grid i2)characteristics. The gain control by the screen grid isin a sense `toop- Ypose-.thesignal carrier variations due toamplitude modulationVwhich would .otherwise occur followingicurve '3 in Fig. .4. vAlthoughtheapeakvalue ofplate current Jhas `been-increased in Fig. 5b over 'Fig5a, the average plate current tends to .re-

main uniform because .the average plate current, under the conditions`0i Fig. 5b, will 4be only `of the order oi 30% of peak value,ascompared with a much larger Apercentage in Fig. 5a.' This is largely dueto the breaks in the plate-current Iias the negativegrid .potentialreaches ori-exceeds cutoff value.

In Fig. 5c Athere are shown the curves a. vand b".,for strong signalreception. The ltube .now

`acts in the manner-of a class C amplier, and the control by thevariable screen voltage-is Aagain shown. It will vbe notedthat `theplatecurrentcontrol grid characteristichasbeen raised further to theposition B', and the ybias .line X has been .moved still.furthernegative The rise-in -the plate current peaks -is compensated byincreased time spacing between -the plate current pulses. The vshadedarea again illustrates the `decreased plate current which wouldl resultif resistor fR were omitted from lthe screen grid/circuit.

Summing -up the actionofithe limiter :tube Figs. V5a,=5l7-and 5c showthat as the fapplied signal amplitude increases, the average screencurrent'fdecreases by reason of the increased negative bias on grid 2.The voltage drop across the screen resistor R is dependent on theaverage screen current `flowing through the res'istorff-obnsecuent1ywhen the average screen-current decreases,.`the effective screen voltageincreases by virtue of a decrease in voltage drop across the screenresistor. This tends to increase the average screen current. The higherscreen voltage, also tends to bring up the average plate current. Theoverall action is such as to tend to maintain a constant average platecurrent. This, in turn, will tend to produce a signal frequencycomponent of the plate cur- .rent that is uniform for all inputamplitudes above the threshold value. So far as the relations betweenthe R, C networks in the control grid circuit and screen grid circuitare concerned, it may generally be stated that the time constant in thescreen grid circuit is relatively faster than that in the control gridcircuit. Generally speaking both time constant networks may be chosenfrom a range of the order of 1 to 10 microseconds.

While I have indicated and described several systems for carrying myinvention into eiect, it will be apparent to one skilled in the art thatmy invention is by no means limited to the particular organizationsshown and described, but that many modifications' may be made withoutdeparting from the scope of my invention.

What I claim is:

1. In combination between a source of frequency modulated wave energyand a frequency Vdiscriminator circuit having a predetermined responsecharacteristic, an amplitude limiter tube including a cathode, signalgrid, plate and auxiliary cold electrode adjacent the plate, means forapplying a positive potential to each of the auxiliary cold electrodeand the plate, means in circuit with each of said signal grid andauxiliary cold electrode for providing a fast-acting gain control overthe limiter tube for carrier amplitude variations in excess of apredetermined value, and means in circuit with the auxiliary electrodefor providing a controlled minimum voltage therefor.

2. In combination between a source of frequency modulated Wave energyand a frequency discriminator circuit having a predetermined responsecharacteristic, an amplitude limiter tube including a cathode, signalgrid, plate and auxiliary coldelectrode adjacent the plate, vmeans forapplying a positive potential to each of the auxiliary cold electrodeand the plate, means in circuit with each of said signal grid andauxiliary cold electrode Vfor providing a fast-acting gain control overthe limiter tube for carrier am.. plitude variations in excess of apredetermined value, said last means consisting kof a resistor and acondenser cooperating to provide a relatively short time constant, and aseries resonant circuit, tuned to the center frequency of said waveenergy, electrically connected to each resistor and associate condenserfor rendering more rapid each time constant.

3. In a system for receiving frequency modulated carrier wave energy, anamplitude limiter tube provided with a cathode, a signal grid, a plateand a screen grid interposed between the signal grid and plate, a signalinput circuit connected between said signal grid and cathode, afrequency discriminator comprising a primary circuit and a secondarycircuit tuned to a common frequency, said primary and secondary circuitsbeing coupled to impart a substantially double-bumped response curve tothe primary circuit, means for establishing said plate at asubstantially. 'constantpositive potential of relatively high magnitude,a Vfirst Qresistorand con- 'denser vnetwork inV circuit with saidsignal. grid and cathode providingt a. relatively short time constant, asecond resistor and vcondenser network in circuit with said screen gridfor providing a relatively shorter time constant than the lfirst timeconstant and auxiliary Ameans in circuit with said screen grid providinga controlled minimum voltage therefor. Y Y

. .4. In combination between a source of frequency modulated wave energyand a frequency discriminator circuit having apredetermined responsecharacteristic, 'an amplitude limiter tube including a cathode, signalgrid, plate and auxiliary cold electrode adjacent the plate, means forapplying a positive potential to each of the auxiliary coldrelectrodeand the plate, means in circuit with each of said` signal grid andauxiliary cold electrode for providing a fast-acting gain control overthe limiter tube for carrier amplitude variations in excess of apredetermined value, and diode means in circuit with said auxiliary coldelectrode for providing a controlled minimum voltage therefor.

5. In combination, between a source of frequency modulated wave energyand a frequency discriminator circuit having a predetermined responsecharacteristic, an amplitude limiter tube including a cathode, signalgrid, plate and aux.. iliary cold electrode adjacent the plate, meanscoupling said grid to said source, means for applying a positivepotential to each of the auxiliary cold electrode and ,the plate, meansin circuit with each of said signal grid and auxiliary cold electrodefor providing a fast-acting gain control overthe limiter tube platecurrent, for carrier amplitude variations in excess of a predeterminedvalue, said last means consisting of a resistor and acondenserco-operating to provide a relatively short Atime constant,Y andmeans connected to each resistor to render more rapid each timeconstant. 5

.6. In combination between a source of frequency modulated wave energyand a frequency discriminator/circuit having a predetermined responsecharacteristic, an Aamplitude limiter tube including a cathode, signalgrid, plate and auxiliary coldV electrode adjacentA the plate, meanslfor applying a positive potential to each of the auxiliary coldelectrode and the plate, means in circuit with each of said signal gridand auxiliary cold electrode for providing a fast-acting gain controlover the tube space current for carrier amplitude variations in excessof a predetermined value, said frequency discriminator cons1sting of aprimary circuit anda secondary circuit coupled to impart a double-humpedchar- REFERENCES CITED The following references areA of record in thefile of this patent:

UNITED STATES PATENTS Name Date Dome Dec. 9, 1941 Number y

